1. Field of the Invention
The present invention relates to a method and apparatus for performing buffer status reporting (BSR), and more particularly, to a method and apparatus for deciding a BSR control element format when performing BSR.
2. Description of the Prior Art
Long Term Evolution wireless communication system (LTE system), an advanced high-speed wireless communication system established upon the 3G mobile telecommunication system, supports only packet-switched transmission, and tends to implement both Medium Access Control (MAC) layer and Radio Link Control (RLC) layer in one single communication site, such as in Node B (NB) alone rather than in NB and RNC (Radio Network Controller) respectively, so that the system structure becomes simple.
In LTE, MAC provides transmission services via a plurality of logic channels. To manage uplink resources, when a user equipment (UE) performs uplink (UL) transmission, the network can divide or map logic channels of the UE into at most four groups based on priorities, types, and so on. The priorities are corresponding to values 1 to 8, and assigned by a higher layer, the radio resource control layer. Besides, the 3rd Generation Partnership Project, 3GPP, introduces a Buffer Status Reporting (BSR) procedure, which is used to provide the serving NB (or enhanced NB) with information about the amount of data in UL buffers of the UE. In the BSR procedure, the UE uses a MAC PDU (Protocol Data Unit) to carry a BSR control element, so as to report information about the amount of data in the UL buffers to the network. Accordingly, the network can determine the total amount of data available across one or all logical channel groups.
In the current specification, there are three types of BSR procedures for different triggering events, a regular BSR, a periodic BSR and a padding BSR. The regular BSR is triggered when UL data arrives at the UE transmission buffer and the UL data belongs to a logical channel with higher priority than those for which data already existed in the UE transmission buffer, or is triggered when a serving cell change occurs. The periodic BSR is triggered when a periodic BSR timer expires. The padding BSR is triggered when UL resources are allocated and an amount of padding bits is equal to or greater than the size of the BSR MAC control element.
Besides, according to different requirements, the BSR MAC control elements can be categorized into two formats: short and long. A short-format BSR control element is 1-byte long, and has 8 bits, where the former 2 bits indicate one logic channel group which buffer status is being reported, and the remaining six bits indicate the amount of data available across the logic channel group. A long-format BSR control element is three-byte long, and utilized for reporting data amount of the UL buffers of all the logic channel groups. Detailed description of the two formats can be found in related protocol specifications, and not narrated herein.
For regular and periodic BSR, the BSR control element format is determined by the UE according to whether more than one logic channel groups has buffered data when the BSR is transmitted. If only one logic channel group had buffer data, the short-format BSR control element is reported; otherwise, if more than one logic channel groups have buffered data, the long-format BSR control element is reported.
However, based on the related protocol specifications, when UL resources for new transmission is available to perform BSR after the regular or periodic BSR is triggered due to a serving cell change or expiration of the periodic BSR timer, for example, if every logic channel group of the UE exists no buffered data, i.e. the amount of data in the UL buffers of every logic channel group is 0, the prior art does not disclose which kind of the BSR control element format shall be used in this situation.